Both transmitting and receiving antennas require transmission lines. There are two basic types of transmission lines: A first type using two or more parallel conductors, and a second type in which one of the conductors is tube-shaped and encloses the other conductor, so called popular coaxial line. With parallel conductors, since the spacing between the two conductors is relatively small, radiation loss can be prevented where the electro-magnetic field from one is balanced everywhere by an equal and opposite field from the other. However, such parallel conductors are susceptible to receiving radiation, altering the carried signal undesirably, and possess other undesirable qualities. Thus, a shielded coaxial cable is preferred and in common usage.
However, with coaxial cable, the current flowing on the inner conductor, although balanced by an equal current flowing in the opposite direction on the inside surface of the outer conductor does not preclude other current flow on the outer skin of the outer conductor. For this reason, coaxial cable is inherently unbalanced.
It is not possible to connect an unbalanced feeder such as a coaxial transmission line to a balanced antenna, and maintain zero potential on the outside of the unbalanced feeder. But it is still current practice to use coaxial line and the very undesirable effects of such currents are tolerated. Some methods to reduce such effects are in use, such as balun (balanced to unbalanced) transformers, but at best they are compromises.
Even when coaxial cable is connected to the physical center of an antenna which is symmetrical about one or more axes, the coaxial cable will induce an imbalance in the operating characteristics of the antenna. Currents flow on the outside of the outer conductor by voltages appearing at the antenna terminal to which it is connected. These currents give rise to unwanted radiation from the transmission line itself, since the field due to them cannot be cancelled by the field due to the current flowing in the inner conductors, which is contained entirely within the outer conductor of the coaxial line and cannot penetrate beyond it.
Stated broadly, that unbalance is caused by the fact that the outside of the outer conductor is not coupled to the antenna in the same way as the inner conductor and the inside of the outer conductor. The overall result is that current will flow on the outside of the outer conductor of the transmission line.
The existence of the unwanted radiation field around the coaxial line will modify the pattern of the antenna. Its input impedance is limited in bandwidth due to the coupling existing between the coaxial line and antenna and will represent a loss of energy by radiation in undesired directions and polarizations. It is also a potential source of TV and broadcast interference and radio frequency feedback problems within the transmitting or receiving facility. Thus it is desired to drive a balanced antenna directly from an unbalanced coaxial transmission line and to prevent unwanted antenna currents flowing back on the outside of the coaxial line.
Various attempts have been made in the past to overcome the inherent effects of using coaxial cable while being mindful of the above problems. A balun is a known technique for balancing the area of conjunction between the feed line and the antenna input. By making this junction balanced, currents of equal amplitude but out of phase will still flow on the outside of the coaxial cable. However being out of phase, since it comes from both halves of the antenna, it cancels itself out and has very little if any measurable effect.
A lumped circuit at the end of the transmission line and antenna junction may be used to provide an unbalanced to balanced connection. However the match depends entirely on the electrical values of the coils and capacitors, and due to their high Q values, the balanced condition bandwidth is very narrow. A similar lumped circuit matching device located at the end of the transmission line remote from the antenna will match the line impedance to the transmitter or receiver, but does nothing for the antenna radiation current flow and radiation on the outer coaxial conductor, line loss or the SWR band width of the antenna.
Another known technique takes advantage of the physical characteristics of coaxial cable and involves placing another tube of appropriate length either over the coaxial cable or parallel to it. The theory involves choking by cancelling the current which travels on the outer surface of the outer coaxial conductor with an opposite current carried on the tube which is either coaxial thereto or parallel thereto. This solution is effective only at one frequency and only between it and the antenna junction, which distance is usually a very small percentage of the total transmission line length.
Still another technique involves the use of coax line configured as a radio frequency choke formed by coiling several turns of the feed line at the point of connection to the antenna. It should be noted that the effectiveness of this type of choke decreases at higher frequencies because of the distributed capacitance among the turns. It suffers from the same lack of total length of transmission line balance as the quarter-wave tube-line choke described above.
In addition, the following prior art citations are listed to show the state of the art further and are submitted in direct response to applicant's acknowledged duty to disclose prior art:
______________________________________ 3,074,064 Pickles January 15, 1963 3,541,570 Onnigian November 17, 1970 4,433,336 Carr February 21, 1984 4,479,130 Snyder October 23, 1984 3,618,110 Solberg November 2, 1971 3,594,807 Tanner July 20, 1971 4,630,061 Hately December 16, 1986 4,617,571 Choquer October 14, 1986 4,254,422 Kloepfer March 3, 1981 2,817,085 Schwartz, et al. December 17, 1957 2,691,730 Lo October 12, 1954 1,715,433 Stone June 4, 1929 1,643,323 Stone September 27, 1927 Publication Hy-Gain May 1987-1988 Publication Cushcraft Corp April 1988 ______________________________________
Pickles teaches the use of a self-supporting dipole antenna with a balun transformer. The coaxial cable passes through one arm of an axial support member which is essentially disposed within a radiator. The coax is then placed on a top surface of the radiator and its center conductor extends to an adjacent radiator fixed thereto on the sheath of a further coax cable, that extends the length of the second radiator. Alternatively, the center conductor can be coupled to a central conductor of a second transmission line and includes material having a high dielectric constant surrounding the central conductor.